1. Field of the Invention
The present invention relates to a throttle control apparatus mounted on an internal combustion engine, and more particularly to a throttle control apparatus which controls an opening angle of a throttle valve by a driving source such as a motor activated in response to operation of an accelerator operating mechanism, and which enables the accelerator operating mechanism to directly control the opening angle of the throttle valve, when the driving source is inoperative.
2. Description of the Related Art
Conventionally, an accelerator operating mechanism has been mechanically connected to a throttle valve, whereas an apparatus for opening and closing the throttle valve, or controlling an opening angle of the throttle valve by a driving source such as a motor in response to operation of an accelerator pedal has been proposed recently. For example, Japanese Patent Laid-open Publication No. 2-204641 disclose a device including means for opening and closing a throttle valve, second driving means driven by a driving source in response to operation of an accelerator operating mechanism, and clutch means for connecting or disconnecting them.
The device is so arranged that when the driving source operates abnormally, the clutch means, which connects the throttle opening means and the second driving means driven by the driving source, will be disconnected, and then the throttle opening means will be driven by first driving means, if the accelerator operating mechanism is operated more than a predetermined amount, to obtain a desired opening angle of the throttle valve. More particularly, in the case where an electromagnetic clutch mechanism constituting the clutch means is de energized, when the accelerator pedal is depressed more than the predetermined amount, it will be mechanically connected with the throttle opening means to obtain the desired opening angle of the throttle valve.
The device disclosed in the above-described publication may perform a so-called traction control. However, if a driver of an automobile depresses the accelerator pedal more than the predetermined amount in the traction control mode, the throttle valve will be opened irrespective of the traction control mode. In this case, if the throttle valve has been required to be fully closed in the traction control mode, the desired acceleration slip control will not be made.
In order to avoid this problem, the device may be structured as follows. First of all, a supporting member is fixed to an extending portion of a throttle shaft to be held at a certain position thereof and restricted from moving in the axial direction thereof. On the extending portion, a rotor is mounted rotatably, and a movable member is mounted between the rotor and the supporting member movably in the axial direction of the throttle shaft. The movable member and the supporting member are connected by a connecting member, e.g., a leaf spring, to bias the movable member toward the supporting member. At a position facing the rotor, disposed is an electromagnetic coil, which is arranged to attract the movable member toward the rotor when energized. The movable member is provided with a pin which extends in parallel with the axis of the throttle shaft. Further, there is provided a driving member which is mounted rotatably about an axis parallel with that of the throttle shaft, and which has an end face to be engageable with the pin perpendicularly to the axis of the throttle shaft. In this case, provided that the axial length of the pin is set to such a length that the pin can be engaged with the end face of the driving member only when the movable member is positioned at the side of the supporting member, as long as the rotor is rotated by the driving force of the driving source, the movable member will not be prevented from rotating, even if the driving member is rotated in response to operation of the accelerator operating mechanism, because the driving member is positioned not to be engaged with the pin.
However, in the case where the electromagnetic coil is de-energized when the driving member is positioned on a line extending axially from the tip end of the pin in the throttle control mode, the movable member might be prevented from being moved toward the supporting member for returning to its initial position, with the pin engaged with the driving member. For example, in the case where the accelerator pedal is depressed largely in the throttle control mode to cause slip of driving wheels so that the acceleration slip control is initiated, if the electromagnetic coil is de-energized by some reason, the pin will contact the driving member to prevent the movable member from returning to its initial position. Depending upon the dimensional relationship among them, therefore, the movable member and rotor might not be disengaged, unless the accelerator pedal is returned to its initial position. Consequently, the driving source and the throttle shaft will be maintained to be engaged with each other, so that the desired acceleration slip control might not be made.
Further, in the case where a single pin is fixed to a position remote from the axial center of the movable member, and the movable member is arranged to be engageable with the rotor at a position remote from the axial center, when the driving member engages with the pin to transmit a driving force thereto in accordance with the accelerator operation, the movable member will be forced to be inclined to its rotational axis, so that the movable member might engage with the rotor. If the movable member is inclined to engage with the rotor, they might be maintained to be engaged with each other, even after the electromagnetic coil is de-energized due to abnormality or the like. In this case, the throttle valve will not be controlled, even if the accelerator operation is made. Whereas, this will not be caused, provided that the clearance between the movable member and the rotor is set to be large enough. In this case, however, a large electromagnetic force will be required, so that not only the large clearance but also a large space for the electromagnetic coil will be needed. Therefore, the apparatus as a whole will become much larger in size than the proposed device in that publication. Although the above described case may rarely happen, it is desirable to provide a structure which will never cause such case, rather than leaving the case as a matter of design.